Troubleshooting nullmailer is pretty hard, in this case only "Unspecified temporary error" was actually logged. A helpful page with troubleshooting tips is Nullmailer Landmine Map, which helped me find the source of this problem.

To reach the above conclusion I killed the nullmailer daemon (service nullmailer stop is a nicer way) and started it manually. It outputs unknown option string: '--starttls' (and the same when trying --tls). Nullmailer uses the executable /usr/lib/nullmailer/smtp to send mail and the version in the Ubuntu 12.04 package only accepts these options:

root# /usr/lib/nullmailer/smtp -h
usage: smtp [flags] remote-address < mail-file
Send an email message via SMTP
-p, --port=INT Set the port number on the remote host to connect to
--user=VALUE Set the user name for authentication
--pass=VALUE Set the password for authentication
-d, --daemon use syslog exclusively
-s, --syslog use syslog additionally
--auth-login Use AUTH LOGIN instead of AUTH PLAIN in SMTP
-h, --help Display this help and exit

The conclusion is that the Nullmailer package can not be used with Google's SMTP server due to lack of including the necessary options when compiling the binares. Another conclusion is that Nullmailers documentation is lacking as well as its error reporting.

You want to have your email traffic to be SSL encrypted. Nullmailer
supports this since version 1.10. Ubuntu 12.04 ships with Nullmailer
1.05-1. To get the latest Nullmailer for Ubuntu do from my Ubuntu PPA (see prior blog post about how I backported Nullmailer to Ubuntu
12.04)